In wireless communication systems offering half duplex group call services, only one member of the group is permitted to transmit at a time while all other group members receive communications. Those skilled in the art will recognize that the transmit mode is a mode in which the user is talking or sending data while the receiving mode operates in a passive listening state. Once a talking group member stops transmitting, any of the other group members that desire to reply, can press a push-to-talk (PTT) button on their transceiver to request to transmit. If more than one group member wishes to transmit, the user that was first to request this status is first granted—while any later requests are denied. This is illustrated in FIGS. 1 and 2.
Prior art FIG. 1 illustrates a half duplex radio system 100 where each of transceivers 101, 103 and 105 are controlled by network control entity 107 in a communications network 109. FIG. 2 is a diagram showing how a transceiver 103 who is first to submit a transmit request is granted that request. In this illustration, the PTT on transceiver 103 is pressed, a transmit request is transmitted 201 to the network control entity 107. The network control entity then grants the transmit request 203 to the transceiver 103. If a subsequent transmit request 205 is made by the transceiver 101, the network control entity 107 will deny that request 207 and the transceiver 101 will be unable to transmit on the network until the PTT on transceiver 103 is released and stops transmitting 209.
Hence, this first-come, first-served, approach works well when all members of the group have a similar response time to their PTT requests from the network. However, the first-come, first-served, floor control does not work well if some members of the group have a significantly slower PTT request response from the network. It can be very difficult for a group member, with a slow response to a PTT request time, to get an opportunity to talk by the time the slow request is received by the network. This occurs since some other members of the group may have already started to transmit in which case this delay or latency is detrimental to the slower user.
As seen in prior art FIG. 3, one situation where the PTT request response time is particularly problematic occurs when using two networks 300. In this example, some members of the group are part of one wireless network 109 (i.e., an Association of Public Safety Communications Officials (APCO) network) and other members of the group are interconnecting from a second network 303 (i.e., a cellular telephone network). In this illustration, a cell phone 301 is operating in a cellular network 303 and the APCO control and bearer (voice) information is carried over an Internet protocol (IP) data connection on the cellular network 303. The IP data connection is carried through a gateway 305 which accesses the APCO network 109. The problem associated with this type of network interconnect arrangement is that it can introduce a very significant PTT request delay to the cell phone 301 over the cellular telephone network 303 that is attempting to communicate in the APCO network 109. FIG. 4 illustrates another timing diagram 400 showing how the messages are sent from two separate communications networks. In this illustration, a transmit request 401 is sent from transceiver 103 to the network control entity 107 where a transmit request message 403 is subsequently granted and from the network control entity to the transceiver 103. In view of the PTT latency delay, even if the cell phone 301 transmits a request before the transceiver 103, it is often delayed such that the request 405′ will be denied 407/407′ by the network control entity 107 since it was received after the request to transceiver 103 was granted. Accordingly, there should be some method to allow a user using the cell phone 301 to gain access to the first network in view the latency in the transmit request.
The prior art also discusses various approaches for dealing with transmit request latency. U.S. Patent Publication No. 2005/0041625, which is herein incorporated by reference, describes an approach for reducing PTT latency in interconnected code division multiple access (CDMA) networks by selecting between two call establishment strategies based on the network signaling capabilities. A slow guaranteed setup is used if it is necessary to allocate a traffic channel to communicate with a Group Communication Server (GCS). A faster optimistic strategy is used if the network permits the subscriber to send control data to the GCS without first allocating a traffic channel. This approach allows existing control messages to be sent faster in an interconnected network environment, rather than making use of a new type of control message to gain floor control in a different fashion.
U.S. Patent Publication No. 2004/0190496, which is herein incorporated by reference, discloses a method for interconnecting cellular systems to an integrated digital enhanced network (iDEN) system to provide group call services. A Voice Signaling Gateway (VSGW) converts non-iDEN signaling to iDEN signaling, allowing non-iDEN subscribers to participate in group calls. This approach does not extend the existing iDEN group call setup protocol to provide a talk-reservation capability.
Finally, U.S. Patent Publication No. 2003/0235184, which is also herein incorporated by reference, adds extensions to real time protocol (RTP) message headers for group call floor control. In this approach, a request to talk is denied if another member of the talk group is currently talking.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.